True static equilibrium and effects on time

[atnu8]

If an object is truly still in space, how would it perceive time?

If light is moving so fast that time has completely stopped, then something that is completely still should see the end of the universe no? is there a limit on the speed of time?

 

[Ibix]

If an object is truly still in space, how would it perceive time?

There is no such thing. Speed is always a relative concept and you can always consider yourself "at rest" or "moving". As physicists, usually we choose the option that makes the maths easier.

If light is moving so fast that time has completely stopped

It isn't. Proper time is not defined along null curves (the technical term for the paths light follows in vacuum). "Time stops at the speed of light" is nonsense promulgated by popsci trying to use maths that's only valid for speeds below that of light to describe something moving at the speed of light.

s there a limit on the speed of time?

It always passes at one second per second for everybody. It's always other people's clocks that tick slowly.

There is no absolute concept of speed. Right now you may consider yourself at rest, or moving at 0.999999...9c as measured by a passing neutrino. It has no effect on your perception of time.

 

[Herman Trivilino]

If an object is truly still in space, how would it perceive time?

There's no way distinguish between being at rest and moving in a straight line at a steady speed. The two states are equivalent. It's hard to answer your question because I don't know how an object would perceive time. People do, and so the way you would perceive time is the same way you do so now.

If light is moving so fast that time has completely stopped,

That's a common but erroneous notion. At light speed the passage of time is not defined. Which is different from the passage of time being zero.

 

[cianfa72]

There's no way distinguish between being at rest and moving in a straight line at a steady speed. The two states are equivalent.

The notion of traveling along a straight line at constant velocity has different meaning depending on the "physics framework" used. The best/invariant one is the notion of proper acceleration, i.e. the acceleration measured by an accelerometer attached to the body under investigation. Motion in straight line at constant velocity is interpretated as zero proper acceleration.

 

[Dale]

If an object is truly still in space, how would it perceive time?

You are currently truly still in your reference frame. How do you currently perceive time?

 

[atnu8]

Thank you all for the replies, I feel like I may be getting the idea..

The perception of time never changes for the observer, time differences can only be perceived if an observer has a change in acceleration in relation to another observer?

You are currently truly still in your reference frame. How do you currently perceive time?

Just trying to wrap my head around the whole 'reference frame' thing. According to Planck Collaboration et al. (2018) the earth is hurtling through space at 369.82±0.11kms−1.
Does this speed have no influence on our perception of time?

If the earth is speeding up in this direction, would time be slowing down for us or can the difference only be noted if there is something to reference against? (like a copy of the earth that wasn't speeding up)

 

[jbriggs444]

Thank you all for the replies, I feel like I may be getting the idea..

The perception of time never changes for the observer, time differences can only be perceived if an observer has a change in acceleration in relation to another observer?

Experiment shows that an accelerating clock continues to keep accurate time. This is the "clock hypothesis". It is tested by measuring the lifetime of particles traversing an accelerator ring at high speeds and correspondingly high accelerations.

Just trying to wrap my head around the whole 'reference frame' thing. According to Planck Collaboration et al. (2018) the earth is hurtling through space at 369.82±0.11kms−1.

This is the speed of the Earth relative to a frame in which the cosmic microwave background radiation would appear to be isotropic.

Does this speed have no influence on our perception of time?

No. Our clocks still advance at one second per second.

If the earth is speeding up in this direction, would time be slowing down for us or can the difference only be noted if there is something to reference against? (like a copy of the earth that wasn't speeding up)

Yes, the only way you can measure a clock's rate is by comparing it to another clock. The details of how that comparison is done will affect the measured result.

Without specifying the measurement details, a broad statement like "our clocks are running slow" would not be specific enough to be meaningful.

There are scenarios where accelerating clocks see time dilation effects. For instance, gravitational time dilation. Which also applies in an accelerating elevator. In these situations, there is a natural way to compare clocks. The "bottom" clock will run slow while the "top" clock will run fast.

 

[Ibix]

According to Planck Collaboration et al. (2018) the earth is hurtling through space at 369.82±0.11kms−1.
Does this speed have no influence on our perception of time?

That's a speed relative to a particularly useful sense of "at rest", one so prevalent in cosmology that people talk about speed relative to it just as casually as we talk about "doing 30mph" when (if we were pedantic physicists all the time) we ought to say "30mph with respect to the Earth's surface". In either case, it's perfectly ok to say you are at rest and the thing we usually call "stationary" is moving. There's no physical test that will prove you wrong or right.

If you are wondering how it works that we can all say that everybody else's clock is running slow, it turns out to be due to different assumptions about what "now" means. It gets messy quite quickly in curved spacetime, but is fairly easy to explain in flat spacetime if you are interested.

 

[cianfa72]

In either case, it's perfectly ok to say you are at rest and the thing we usually call "stationary" is moving. There's no physical test that will prove you wrong or right.

Yes, however the person who says "I'm stationary" must measure for themself zero proper acceleration.

 

[PeterDonis]

the person who says "I'm stationary" must measure for themself zero proper acceleration.

No, this is not a requirement. For example, you're perfectly justified in saying you're stationary as you type your posts here. But you don't have zero proper acceleration (unless you're posting from the International Space Station, which I assume is not the case).

 

[cianfa72]

No, this is not a requirement. For example, you're perfectly justified in saying you're stationary as you type your posts here. But you don't have zero proper acceleration (unless you're posting from the International Space Station, which I assume is not the case).

Ah ok, so here we are taking just a kinematic viewpoint/description, no dynamic is involved.

 

[Herman Trivilino]

Just trying to wrap my head around the whole 'reference frame' thing. According to Planck Collaboration et al. (2018) the earth is hurtling through space at 369.82±0.11kms−1.
Does this speed have no influence on our perception of time?

Is this a rehash of your original question? Look again at the answer I gave you.

The issue you are having trouble wrapping your head around is the fact that motion is relative. Relative to your desk you're motionless. The motion you quote above is relative to the sun. But we're also in motion relative to other things, like the center of our rotating galaxy. Or the center of Earth.

Do a google search for "Galileo in a ship hold". He does a good job of explaining it. He was attempting to dispel the notion that Earth must be at rest because we cannot detect its motion. For example, Earth rotates once per (sidereal) day. Consequently, at my latitude I move a distance of about 24 000 miles in 24 hours. That's 1000 mi/h. A wall just west of me is moving towards me at that speed. If I drop my pen why does the wall not slam into it before it can hit the floor? Galileo's contemporaries regarded the fact that this doesn't happen as proof that Earth is not moving. Galileo dispels that argument.

This idea is embedded in the 1st Postulate of Einstein's relativity. Since Galileo's day it's place in the hierarchy of physics has increased, and is now called the Principle of Relativity.

This idea forms part of the foundation for the issues you are struggling with. You'll do yourself a favor by spending time absorbing it.

 

[pervect]

Thank you all for the replies, I feel like I may be getting the idea..

The perception of time never changes for the observer, time differences can only be perceived if an observer has a change in acceleration in relation to another observer?

Just trying to wrap my head around the whole 'reference frame' thing. According to Planck Collaboration et al. (2018) the earth is hurtling through space at 369.82±0.11kms−1.
Does this speed have no influence on our perception of time?

If the earth is speeding up in this direction, would time be slowing down for us or can the difference only be noted if there is something to reference against? (like a copy of the earth that wasn't speeding up)

I would suggest learning about quantities, called invariants, that are independent of the frame of reference. In special relatiavity, his would be, for instance, the Lorentz interval. This would be discussed in textbooks such as "Space time physics" by E.F. Taylor. An older edition is available for free on his website - this is a standard textbook with a rather informal and chatty style.

See for instance https://www.eftaylor.com/spacetimephysics/

The common definition of distance intervals and time intervals do change with one's reference frame, but there is a way of combining them that does not.

"The Parable of the Surveyor", the first chapter of the reference I just gave, is perhaps a bit longwinded but describes how north-south and east-west are part of a larger concept in surveying. It is worth thinking about why we regard north-south and east-west as being combined into a larger structure, rather than two separate entities.

This sounds simple enough - and it is - but there is one obstacle that people stumble over all the time, and something that Einstein had to struggle with to formulate the special theory of relativity. This is the notion that the idea of "at the same time", or "now", depends on the frame of reference.

Many, many people find this very hard to grasp, and it is a notorious obstacle to understanding special relativity. It's unclear if bringing this up now is the best approach, but it just doesn't make sense that switching from distance and time intervals (which vary with the reference frame) suddenly become frame independent if they don't, somehow, interact with each other. It turns out that while space and time intervals do vary with the reference frame, a fairly simple combination of them does not.

The actual math formula is actually simple - if dx is a distance interval, and dt is a time interval c^2 dt^2 - dx^2, and it's inverse dx^2 - c^2 dt^2, are invariant and independent of the frame of reference, while dt and dx are not independent.

This speciflally means that if dt equals zero in one frame of reference it may not be (and usually is not) zero in a different frame of reference. Hence, the phenomenon that is called "the relativity of simultaneity".

This is a simple example with only one dimension of space, but that's more than good enough to get started. Note that for any two points connected by a beam of light, this formula gives Lorentz interval of zero. That' where the remarks about "null intervals" come from.

It's a LOT easier to keep tract of things that don't change with the frame of reference than it is to keep tract of things that do deped on the frame of reference. For one thing, one winds up tediously haveing to give the exact details of what frame of rerference one is using. This is doable, and sometimes can't be avoided, but it is in general much easier to talk about things that are the same in all frames of references - things that are invariant.

 

[PeterDonis]

so here we are taking just a kinematic viewpoint/description, no dynamic is involved.

"Stationary" is a matter of "kinematics" as you are using the term here. It's just a choice of reference frame. You can do it with any "dynamics" you like.

 

[cianfa72]

"Stationary" is a matter of "kinematics" as you are using the term here. It's just a choice of reference frame. You can do it with any "dynamics" you like.

Ok, so in the context of Newtonian physics let's pick the frame in which I am at rest. What if this isn't inertial? To do dynamic w.r.t. it, one is forced to add inertial forces appearing to act on all objects (including me) in this frame.

I'm not sure whether from a dynamic perspective I've the "right" to say "I'm stationary" though.

 

[PeterDonis]

so in the context of Newtonian physics let's pick the frame in which I am at rest. What if this isn't inertial?

The frame in which you are at rest right now is not inertial. You are stationary in this frame--because you are at rest in the frame. Why is that a problem? We all use such a frame in our daily lives all the time.

To do dynamic w.r.t. it, one is forced to add inertial forces appearing to act on all objects (including me) in this frame.

Well, of course. That's how a non-inertial frame works. Why does that make it a problem to use the word "stationary" for an observer at rest in the frame?

I'm not sure whether from a dynamic perspective I've the "right" to say "I'm stationary" though.

Why wouldn't you?

I simply don't see why any of this is an issue.

 

[atnu8]

I would suggest learning about quantities, called invariants, that are independent of the frame of reference. In special relatiavity, his would be, for instance, the Lorentz interval. This would be discussed in textbooks such as "Space time physics" by E.F. Taylor. An older edition is available for free on his website - this is a standard textbook with a rather informal and chatty style.

See for instance https://www.eftaylor.com/spacetimephysics/

This book is great! I love how it's written. I just read chapter 4: Trip to Canopus and found that extremely enlightening, thank you for sharing.

I appreciate all of you for being patient with me!

How does this sound:

All measurements of time are dilated when moving at high speed relative to an inertial observer.
(Time slows down for moving observers compared to non-moving observers)

Because the speed of light is finite, moving through space causes any physical mechanical processes to take longer.

If a rocket was to counteract the velocity of the earth's orbit around the sun, staying still in relation to the sun but moving parallel with the sun through space, when the earth met with the rocket again in space and completing a full orbit of the sun, the clocks on earth would measure less time elapsed than the clocks in the rocket.

 

[Nugatory]

This book is great! I love how it's written. I just read chapter 4: Trip to Canopus and found that extremely enlightening, thank you for sharing.

You are just cheating yourself out of essential background understanding (which is the really fun part) if you've skipped chapters 1-3.
(Edited to add: I said above that I had learned relativity from that book - actually the first edition, which I still prefer. By far the hardest part was realizing that those first chapters were not "yes of course" obvious stuff that I already knew, but instead a gentle but very firm process of getting me to see that all of my intuitions about time, distance, motion were seriously incomplete).

How does this sound:
All measurements of time are dilated when moving at high speed relative to an inertial observer.
(Time slows down for moving observers compared to non-moving observers)
Because the speed of light is finite, moving through space causes any physical mechanical processes to take longer.

Not right, and a strong hint that you have missed the point.
(You should never say "moving" without also saying what the motion is relative to. This is not a pedantic quibble, the habit is essential to being able to think about spacetime clearly).

Consider: You are sitting on the surface of the earth. Your speed is zero if we use a frame (this is in chapter 2) in which the surface of the earth is not moving - another way of saying that your speed is zero relative to the surface of the earth. Your speed is many kilometers per second if we use a frame in which Mars is at rest. That's two different speeds, so if moving through space changed the speed of physical processes, then all the physical processes in your body would be happening at two different speeds at the same time, which is impossible.

If a rocket was to counteract the velocity of the earth's orbit around the sun, staying still in relation to the sun but moving parallel with the sun through space, when the earth met with the rocket again in space and completing a full orbit of the sun, the clocks on earth would measure less time elapsed than the clocks in the rocket.

Not for the reason you're thinking. It has nothing to do with motion affecting the passage of time, it is because the rocket and the earth have taken different paths through spacetime, the different paths have different lengths, the length of a path through spacetime is measured in seconds by a clock following that path just as the length of a path through space is measured by the odometer of a car following that path. (Correctly calculating the length of that path when the earth is rotating, orbiting, and everyone is affected by gravity is a seriously non-trivial problem... Don't take it on until you understand the simple out and back no-gravity Twin Paradox case).

Time dilation between observers in motion relative to one another is a different thing, the result of relativity of simultaneity, which is in chapter 3.

 

[Herman Trivilino]

(Time slows down for moving observers compared to non-moving observers)

You're still not getting it. If we are moving inertially relative to each other then either of us could validly claim to be at rest and the other in motion. Each of us would observe the other's time to be dilated. A notion that stumped Einstein himself as he was creating his theory. He resolved it when he realized that it could be explained by the fact that simultaneity is relative. He claimed that when the realization came to him it caused him to sit upright when he had been lying in bed.

There is no such thing as "moving through space".

 

[pervect]

This book is great! I love how it's written. I just read chapter 4: Trip to Canopus and found that extremely enlightening, thank you for sharing.

I appreciate all of you for being patient with me!

How does this sound:

All measurements of time are dilated when moving at high speed relative to an inertial observer.
(Time slows down for moving observers compared to non-moving observers)

You seem to be stuck on the idea that "time slows down". One can ask "Slows down relative to what? To me, your words suggest you believe in some sort of "absolute time" in which things can be at "absolute rest", this viewpoint has been presented by you several times and hasn't changed through the thread in spite of our efforts to explain that it's not the viewpoint of special relativity. But I'll repeat for emphasis that this is not the viewpoint of special relativity. This, my take on what you are saying is that when you talk about "time slowing down", you think that the time you measure with your clock slows down relative to some sort of absolute time. While such absolute time is present in Newtonian theory, it is NOT present in special relativity. Therfore, if you want to understand special relativity, you need to abandon this notion of absolute time. This make take some effort.

I'll try and break down the concept of time in special relativity. There are at least two sorts of time in special relativity. One is proper time. THis is the sort of time measured with a clock. A clock can be regarded as "travelling through spacetime", and, a bit like a wristwatch, if the clock is present and two events at different times, it will give a reading as to the time difference between the two events.

The second notion of time is different from the above. It's the notion of "simultaneous events". When a clock is not physically present at both events, one needs such a notion to measure the time interval between said events.

There is possibly a third notion related to time here, the notion of causality. But I won't be going into that in this post.

Separating and disambiguiting the two notions of time (proper time and simultaneity) are I think the most that it is reasonable to address in one post.

I will describe my view of "time dilation", and, if you are capable of drawing space-time diagrams, I'd encourage you to draw them. If you're not capable, I'd encourage learning how to draw them. You'll need the related concepts of "events" and "worldlines".

To do this, you will need both of the concepts I outlined - the notion of "proper time" and the notion of "simuttaneity" or "now". And I'll be using some concepts which are standard in space-time diagram, namely events (which happen at some specific time and place), and worldlines (which are the set of all events that some observer experiences).

Draw two worldlines on a space-time diagram, A and B. We'll regard one worldline, worldine A, as our reference. You might call it "stationary". And the second worldine B is a worldine of "an observer moving relative to A". If you're familir with the distinction, we are talking about "timelike worldlines" here, worldlines that are a one-dimensional set of events that an observer "experiences".

Along worldine A, using a clock, we mark regular proper time intervals. Because it's a proper time interval, it has a length, which we measure with a clock. We'll call this interval t_a.

For every point on worldline A, we find the unique event on worldline B that occurs "at the same time" as the event on worldline A. To do this, we need a notion of simutaneity, of events that occur "at the same time", a concept of now. There's a formal way we do this in specail relativty, we use what we call Einstein's clock synchronization convention, and to apply this convention proeprly, we need to specify a specific frame or observer in which we will apply this convention.

These events that we mark along worldline B turn out to also occur at regular intervals of proper time on worldine B, but the value of this interval (which we again measure with a clock) is t_b, which is in general different from t_a. The ratio of t_b to t_a is then the time dilation.

Einstein clock synchronization is important here, and if you haven't studied it, it's something you need to study to grasp special realtivity. The easiest way to study is is to draw space-time diagrams. To provide some physical insight for the space-time diagrams, you need to know how to measure time intervals (proper time intervals0 on the diagram, how to measure space intervals on the diagram (distances with a ruler), and how to synchronize clocks (the Einstein clock syncrhonization method). You need to be able to do all th ree.

We are perfectly free to exhcange the roles of A and B. When we do so, though, it turns out t he process of Einstein clock syncrhonization depends on our choice. The correspondence of points on worldline A to worldine B is a map. The map between A to B in A's frame is different from the map between A and B in B's frame, so when we interchange the observers, we use a dfferent map. THis is important.

Your second point involves GR and not SR. The short answer is that you got this one right, but discussing it at length would make thyis post too long - it's already probalby presenting too much material.

 

[Herman Trivilino]

Therfore, if you want to understand special relativity, you need to abandon this notion of absolute time. This make take some effort.

In my experience this definitely takes a great deal of time and effort. Reading something like Spacetime Physics by Taylor and Wheeler and working through all the examples and chapter-end exercises requires time and effort. But there's no substitute for it. Reading popsci explanations is more entertaining and requires less mental effort and time. Either way you're left with misconceptions, but the former gives you the tools to resolve them whereas the latter does not.